The aims of the research are to investigate some implications of the general proposition that the DNA of active genes is relatively extended and that compacted heterochromatic chromatin results from a coordinated system of gene inactivation, as follows: 1) Experimental test for presence of genes in extended DNA of interbands of polytene chromosomes. 2) Investigation of the effect of deficiencies in "constitutive heterocromatin" on viability and fertility. 3) Exploration of bristle cell nuclei and mutants which affect bristles, as an example of a differentiating system to be studied at the chromosome level. Drosophila melanogaster will be used as experimental material. Cytogenetic analyses will be made of existing stocks and of new mutants and deficiencies induced by the use of X-rays.